This invention relates to fuel cell systems and, in particular, to control assemblies for controlling fuel cell systems during shutdown and restart.
A fuel cell is a device, which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
A molten carbonate fuel cell (“MCFC”) system operates by passing a reactant fuel gas through the anode of the fuel cell of the fuel cell system, while an oxidizing gas is passed through the cathode of the fuel cell of the fuel cell system. The fuel supplied to the MCFC system is typically a hydrocarbon fuel which needs to be humidified, usually by adding vaporized water or steam, before the fuel enters the anode of the fuel cell. During MCFC operation, the oxidizing gas keeps the cathode of the fuel cell in an oxidizing atmosphere, while the humidified fuel gas keeps the anode of the fuel cell in a reducing atmosphere.
When the MCFC system is shut down, i.e., when the fuel cell is at its operating temperature and the fuel and oxidant gases are no longer supplied to the system, a number of things must occur to protect the system. First, the oxidizing atmosphere must be maintained in the cathode of the fuel cell and the reducing atmosphere must be maintained in the anode of the fuel cell. Second, the humidified fuel present in the fuel lines of the MCFC system needs to be purged in order to prevent condensation, possible Nickel Carbonyl formation and damage to the catalysts in the fuel cell by liquid water.
When the MCFC system is restarted, i.e., when the flows are reestablished after a shutdown, again certain conditions must be present to ensure continued system operation. During restart, the oxidant flow is established before initiation of the fuel flow. Thus, a reducing atmosphere needs to be maintained in the anode of the fuel cell during the time when the regular fuel flow is not available to prevent oxidation of the anode. Additionally, during this time, carbon dioxide needs to be supplied both to the anode gas stream and to the cathode of the fuel cell. This is needed in order to prevent decomposition of the fuel cell electrolyte as well as to prevent fuel cell matrix particle growth at the matrix anode interface.
A number of techniques for controlling a fuel cell system during shutdown have been disclosed. Japanese Patent Application Publication No. 04004570 discloses a fuel cell system in which a standby gas containing mainly hydrogen is supplied to the anode of the fuel cell during a shutdown of the system and when the system is at its normal operating temperature. This prevents oxidation of the anode. Another Japanese Patent Application Publication No. 04324253 describes a fuel cell system having a standby gas used to prevent oxidation of the anode of the fuel cell during shutdown of the system. In this case the standby gas is prepared by mixing a reducing gas with nitrogen.
Japanese Patent Application Publication No. 10289724 discloses another method used during shutdown of a fuel cell system. In this method, an inerting gas made of nitrogen or argon and containing between 1 and 10% carbon dioxide is supplied to the fuel cell cathode to reduce growth of LiAlO2 particles in the electrolyte plate. European Patent Application Publication No. EP01481436 refers to another inerting procedure used during shutdown of a fuel cell system in which water vapor is supplied to the fuel cell anode and an electrolysis reaction is effected by applying an external potential to the fuel cell. Finally, Japanese Patent Application Publication No.10032013 describes a purging method used to control a fuel cell system after a shut down in which purging is accomplished by re-circulating the anode and cathode streams separately and inerting the streams separately.
The systems discussed above, however, do not provide an overall system capable of realizing the conditions described previously as required by a MCFC system during shutdown and restart. An arrangement satisfying these conditions is thus needed for efficient operation of the fuel cell system.
It is therefore an object of the present invention to provide a fuel cell system and method which can satisfy the conditions for efficient operation of the system during shutdown and/or restart.
It is also an object of the present invention to provide an integrated fuel cell system where the appropriate atmosphere is maintained in the anode and the cathode of the fuel cell of the system during shutdown, without physically isolating the two.
It is a further object of the present invention to provide an arrangement and method of the above type which is cost effective and simple.